Combustion air heater for furnaces arranged to minimize corrosion by flue gases



June 13, 1950 J. MARSHALL COMBUSTION AIR HEATER FOR FURNACES ARRANGED T0 MINIMIZE CORROSION BY FLUE GASES Flled July 11, 1946 A H "F TW M "MW m x mm mJ .1 1 n. z m a z? w 1% 0 e a a 3 z B 9 5 I V; w J w a u a 2 9 a 6 2 6 5 wfl; w A I N 1 fly /AW\1||Y.\ Wk M W s 64 5 F10 7 ,l. j 3 5: 1 5 ,0 la I 7 1,11 1/ 3 9% h 2 k Patented June 13, 1950 COMBUSTION AIRHEATER FOR FURNACES ARRANGED 'ro 'MINIMIZE CORROSION BY FLUE GASES Leonard J. Marshall, Tenafly, .N. J assignor, by

mesne assignments, to Combustion Engineering-Superheater, Inc., a corporation of Dela- Ware ApplicationJulyIl, 1946, Serial N0. 682,792

Claims. 1

This invention relates to .heat exchange apparatus and particularly to improvements in air heaters.

In the conventional, air heater the heating gases and the cold air to be heated usually fiow in counter-current relation to each other with the result that heat absorbing surface at one end of the heater is swept by cold air and cooled gases. This results in low metal temperatures of the heat absorbing surface at the cold end where air enters and gas is discharged. In air heaters in which the heating gases are to be cooled to a low temperature to .attain a high-efflciency of heat recovery, the metaltemperatures at the gas outlet end may be so low as to be below the dew point of the gases andthereby cause corrosion due to condensation of moisture in the presence of gases containing sulphur oxides.

An object of this invention is to provide improved means for controlling the metal temperature at the gas outlet end of an air heater so as to maintain a high efilciency of heat recovery. Other objects and advantages of .the invention will appear upon consideration of the following detailed description of an illustrative embodiment of the invention when read in conjunction with the accompanying drawings in which; the single figure represents a sectionalelevation of a steam generating unit embodying the invention.

The invention contemplates providing a boiler that normally burns within the furnace blast furnace gas and coke breeze with means for burning oil to obtain full capacity of the unit should either or both of the other fuels fall short of the required amount or be wholly unavailable.

The steam generating unit comprises a chain grate stoker designated A, a super-imposed .furnace B, superheaters C in the ofitake of said furnace, a steam generating bankD, an economizer E and a tubular air heater The stoker is of the usual traveling grate type in which a grate surface receives fuel from hopper H, in this instance coke breeze. The fuel burns progressively as the grate advances to :the left and the ash is discharged over the rear endinto an ash hopper I2. A long rear arch .I3 is provided over the grate, the forward end of the arch joining with the inclined bottom of the furnace :B. The furnace is enclosed by walls l5, l5 and H and a roof l8 all of which may be cooled by providing them with boiler tubes which are connected into circulation with the generator. Wall contains three groups of burners, the two lower groups 43 being for blast furnace gas and the .upper group being for oil. An arch 2| above the burners directs the gases upwardly into offtake 22 in which are suspended the elements 23 of superheater C. Adjacent the ofitake 22.15 the bank D comprising a lower drum 24 and an upper drum 25 interconnected by a bank of tubes 26 and a steam drum 2! connected to the lower drum 24 by rows of tubes 28 and 2 9 which form achamber for economizer E. Baffles 30 and 3| form two passes through the boiler for the gases leaving the offtake 22, the upper pass including a portion 32-0f the superheater C. Dampers 33 and 34 control the proportional flow of gases over the respective passes to control the superheat.

After leaving the boiler the gases pass downwardly through the economizer E thence through conduit 35 to the upper end of air heater F. The air heater F may be a plate or tubular type and as shown comprises a bank of vertical tubes 36 in parallel arrangement rolled into top and bottom tube sheets .31 and 38. The bank of tubes is enclosed within a chamber formed by side walls 39 and 40, end walls 4| and-topand bottom walls 42 and 43. Within said chamber are provided baflles 44, 45, 46, 41 and 48 so arranged as to cause the air to flow back and forth across the tube bank transversely through serially connected passes 49, 150, 51,52, 53 and .54. An air inlet duct 55, fed through conduit 5511 .from any suitable source -(notshown), is connected to the ends of passes *49 and 53 located near the lower end of the air heater but above the lowermost end pass thereof. An air outletduct 56 is connected to the end of pass 52 at the top of the air heater. A second air outlet duct 51 is connected to the end of pass 54'at the bottom of the air heater. 'I-'he lower air outlet duct '51 connects through ducts 6i and-62 with the stoker A toprovide air for combustionof the coke breeze. -A by-pass duct 58 is connected betweenlower air outlet duct 51 and pass 5n anda damper 66 is provided .in the opening in wall '40. Optionally Joy-pass duct 58 may be extended upwardly to connect .into the upper air outletduct -56. Dampers 59 and it! are providedat the ends of by-pass 58. Partition -48 is provided with a dampered opening .65 between air inlet duct and the lower air outlet duct 51. Below the tube sheet 38 of air heaterF is conduit 63 which is connected to induced draft fan 64.

In the conventional -air heater the downfiow air passes .53 and 54 are not provided and the bottom tube sheet 38 usually constitutes the outer boundary-of the first .air pass at the cold end. In such'a heater the lowest metal temperature of the heating surface is adjacent the gas outlet end or just above tube sheet 38. The minimum tube temperature or coldest spot in the air heater is in those tubes next to the air inlet just above tube sheet 38 and moving in the direction of air flow through the first air pass in the equivalent location of pass 53 the tube temperature increases.

According to the invention the cold airinlet duct of the air heater is moved upwardly away from its former location at the bottom tube sheet to a hotter portion of the tubes, such as at the illustrated location of duct 55, and a portion of the air is caused to flow downwardly over the bottom ends of the heating surface through one or more passes such as 53 and 54' to a separate outlet duct 51. This portion of air is heated in its downward flow and absorbing less heat than would the entire volume of air, thereby eliminates the objectionable cooling below the gasdew point encountered at the bottom of the conventional air heater. The remaining or major portion of the air flows from inlet duct 55 countercurrent to the gas flow, up through the heater passes 49, 50, and 52 and leaves the heater from pass 52 to flow into the upper air outlet duct 56. Although the temperature of the downwardly flowing flue gas leaving theimproved heater at tube sheet 38 is colder than when it passes at the level of partition 41, the air flowing through pass 54 is hotter than'that flowing through pass 53 which results in a hotter metal temperature at the bottom of the air heater than would occur in the conventional air heater having the same heating surface; By manipulating by-pass dampers 59 and/or 68, the amount of air flowing through passes 53 and 54 may be controlled and thereby the metal temperature of the adjacent heating surface.

In case the improved air heater is used in association with a steam generator burning but one fuel, it may be proportioned for maximum heat recovery from the flue gas at a certain load with safe metal temperature at the gas outlet end. At other loads the dampers 59 and/or 60 may be manipulated to control said metal temperature to that at which corrosion due to condensation of portions of the flue gas will not occur.

The improved air heater maybe 'advanta geously used in association with a unit burning several fuels, for example: When burning blast furnace gas together'with coke breeze and/or oil as shown in the figure, the ratio of the products of combustion to air and the quantity of products of combustion increase with increase in blast furnace gas burned. An air heater built for high heat recovery when burning a substantial portion of blast furnace gas in the furnace with the other fuels will when burning coke breeze and/or oil in major proportion or alone, have a relatively colder metal temperature at the gas outlet end which may well approach the temperature at which condensation of portions of the gas occurs which will cause corrosion. As mentioned above the cold air is delivered to the improved air heater where the metal temperature of the heating surface is higher than at the gas outlet end, such as at air inlet duct 55. A major portion of the air then flows from inlet duct 55 in countercurrent' to the gas flow, up through the heater passes 49, 58, 5| and 52 and leaves the heater from pass 52 to flow into the upper air outlet duct 56. The remaining portion of the air flows from inlet duct 55 in parallel current to the gas flow down through the heater passes 53 and 54 and leaves the heater from pass 55 t0 flow into the lower outlet duct 51.

" If there is no use for air in the stoker A, the

damper in duct Bl to the stoker is closed and the air from outlet duct 51 may pass upwardly through by-pass duct 58 to return to the air heater into an upper pass such as pass 5|. Damper 59' in by-pass duct;58 controls'the quantity of air flowing therethrough and therefore through the bottom passes 53 and 54 of the air heater. With dampers 59 and 61 open and dampers 65 and 5B closed the heated fluid from pass 54 may be by-passed around the major part of the exchanger to mix with heated fluid from the upper pass. 52. The quantity of air flowing through these passes 53 and 54 determines the amount of heat given up by the heating surface within these passes and thereby governs the temperature of the metal of said heating surface. When burning blast furnace gas in major portion, damper 59 in by-pass duct 58 will be opened and the heating surface will be efiective throughout the entire length of the air heater. As mentioned above, when burning coke breeze and/0r oil in major portion, the bottom end of the heating surface or gas outlet end of the air heater, will tend to become too cold, and can now be prevented from becoming so by reducing the air flow through passes 53 and 54. In effect the air heater is thereby reduced in size and when the air to passes 53 and 54 is entirely shut off, the effectiveness of the air heating surface substantially ends at partition 41 above pass 53.

Should the stoker require air, damper 59 in duct 58 may be closed or partially so and damper 65 in duct 6| opened thereby forcing the air to flow to the stoker. Should mean to the stoker become too hot, it may be tempered by passing some cold air through by-pass damper 60 into the lower air outlet duct-51. Obviously the by-pass damper 68 may be used to supplement the control of the amount and temperature of the air passing through the passes 53 and 54, whether this air passes up through by-pass duct 58 to the upper portion of the air heater or passes down to the stoker via duct 6|.

While the preferred embodiment of the invention has been shown and described, it will be understood that minor changes in construction, combination and arrangement of parts may be made without departing fromthe spirit and scope of the invention as claimed.

What I claim is:

1. In a heat exchanger comprising a chambe the combination of a plurality of conduits extending lengthwise therethrough, means for flowing a heating gas through said conduits from the gas-entering or high-temperature end of the chamber to the gas-exit or reduced-temperature end thereof, a plurality of battles spacedly positioned between the two chamber ends to form within the chamber a plurality of transverse passes serially extending back and forth across the exterior surfaces of said conduits, an inlet duct for directing a fluid to be heated into each of an intermediate pair of said transverse passes remote from thetransverse passes at the extreme chamber ends, a first outlet duct for the heated fluid connected to the extreme-end pass at said high-temperature chamber end to discharge the heated fluid there received from said interme chamberend to discharge the heated 'fluid there received from said intermediate inlet through the transverse pass series leading to that coolest answer chamber end, said intermediate-location entry of the fluid to belieated serving to safeguard said heating gas against being cooled by thecold coming fluid "to a, temperature below the gas dew point and thereby keeping corrosive constituents of that heating :gas from-condensing on said conduits at said chambers coolest end or elsewhere in the heat exchanger.

2. In a heat exchanger comprising a chamber, the combination of a plurality of conduits extending lengthwise therethrough, means for flowing a heating .gasthroughsaid conduits from the gasentering or high-temperatnreend of the chamber to the gas-exit or reduced-temperature end thereof, a plurality of baflles spacedly positioned between the two chamber ends to form within the chamber a plurality of transverse passes serially extending back and forth across the exterior surfaces of said conduits, an inlet duct for directing a fluid to be heated into each of an intermediate pair of said transverse passes remote from the transverse passes at the extreme chamber ends, a first outlet duct for the heated fluid connected to the extreme-end pass at said hightemperature chamber end to discharge the heated fluid there received from said intermediate inlet through the series of transverse passes leading to that hottest chamber end, a second outlet duct for the heated fluid connected to the extremeend pass at said reduced-temperature chamber end to discharge the heated fluid there received from said intermediate inlet through the transverse pass series leading to that coolest chamber end, a by-pass duct connecting one of said first and second outlet ducts for the heated fluid to an intermediate transverse pass other than those joined with said inlet duct, and damper means in said by-pass duct for controlling flow of heated fluid therethrough, the aforesaid intermediatelocation entry into said transverse passes of the fluid to be heated serving to safeguard said heating gas against being cooled by the cold incoming fluid to a temperature below the gas dew point and thereby keeping corrosive constituents of that heating gas from condensing on said conduits at said chambers coolest end or elsewhere in the heat exchanger.

3. In a heat exchanger comprising a chamber, the combination of a plurality of conduits extending lengthwise therethrough, means for flowing a heating gas through said conduits from the gas-entering or high-temperature end of the chamber to the gas-exit or reduced-temperature end thereof, a plurality of baflies spacedly positioned between the two chamber ends to form within the chamber a plurality of transverse passes serially extending back and forth across the exterior surfaces of said conduits, an inlet duct for directing a fluid to be heated into each of an intermediate pair of said transverse passes remote from the transverse passes at the eX- treme chamber ends, a first outlet duct for the heated fluid connected to the extreme-end pass at said high-temperature chamber end to discharge the heated fluid there received from said intermediate inlet through the series of transverse passes leading to that hottest chamber end, a second outlet duct for the heated fluid connected to the extreme-end pass at said reducedtemperature chamber end to discharge the heated fluid there received from said intermediate inlet through the transverse pass series leading to that coolest chamber end, a first by-pass duct connecting said second outlet duct for the heated fluid to an intermediate transverse pass other than those joinedwith :saidinlet duct, a second byepass ductconneclti-ng said inlet duct to said second outlet-duct, and damper means in each of said first and second hy-passesthe aforesaid intermediatedocation entry-into said transverse passes "of, thefluid to be heated serving to safeguard said-heating ,gas (against being cooled by the cold incoming fluid to atemperature below the gas Edew "point and thereby keeping corrosive constituentspfuthat heatin gas from condensing on said conduits-at said chambers coolest end or elsewhereyin the :heatexchanger.

4. Ina heat exchanger comprising a chamber, the combination ofa plurality of conduits extending lengthwise therethrough, meansfor flowing a heating gas through said conduits from the gas-entering or high-temperature end of the chamber to the gas-exit or reduced-temperature end thereof, a plurality of bafiies spacedly positioned between the two chamber ends to form within the chamber a plurality of transverse passes serially extending back and forth across the exterior surfaces of said conduits, an inlet duct for directing a fluid to be heated into each of an intermediate pair of said transverse passes remote from the transverse passes at the extreme chamber ends, a first outlet duct for the heated fluid connected to the extreme-end pass at said high-temperature chamber end to discharge the heated fluid there received from said intermediate inlet through the series of transverse passes leading to that hottest chamber end, second outlet duct for the heated fluid connected to the extreme-end pass at said reduced-temperature chamber end to discharge the heated fluid there received from said intermediate inlet through the transverse pass series leading to that coolest chamber end, a damper in the discharge portion of said second outlet duct, a by-pass duct connecting said first outlet duct with said second outlet duct on the upstream side of the damper therein, and damper means in said by-pass duct, the aforesaid intermediate-location entry into said transverse passes of the fluid to be heated serving to safeguard said heating gas against being cooled by the cold incoming fluid to a, temperature below the gas dew point and thereby keeping corrosive constituents of that heating gas from condensing on said conduits at said chambers coolest end or elsewhere in the heat exchanger.

5. In combination, a furnace having an offtake for discharge of combustion gases, an air heater comprising a chamber having a gas-entering end connected with said oflftake and a gasexit end remote therefrom, a plurality of conduits extending lengthwise through said chamber to conduct said combustion gases from said gas-entering or high-temperature chamber end to said gas-exit or low-temperature chamber end, a plurality of baffles spacedly positioned between said two chamber ends to form within the chamber a, plurality of transverse passes serially extending back and forth across the exterior surfaces of said conduits, an inlet duct for directing air to be heated into each of an intermediate pair of said transverse passes remote from the transverse passes at the extreme gas-entering and gas-exit chamber ends, a first outlet duct for the heated air connected to the extreme-end pass at said gas-entering or high-temperature chamber end to discharge the heated air there received from said intermediate inlet through the series of transverse passes leading to that hightemperature chamber end, a second outlet duct for the heated air connected to the extreme-end pass at said gas-exit or low-temperature chamber end to discharge the heated air there received from said intermediateinlet through the transverse pass series leading to that low-temperature chamber end, and fuel-burning means within the furnace fed with the heated air from at least one of said first and second outlet ducts, said intermediate-location entry into said transverse heater chamber passes of the air to be heated serving to safeguard said furnace combustion gases in the heater conduits from being cooled by the cold incoming air to a temperature below the combustion gas dew point and thereby keeping corrosive constituents of those gases from conll REFERENCES cl'rEn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,785,334 Black Dec. 16, 1930 1,819,174 Jacobus Aug. 18, 1931 2,363,870

Karlsson et a1 Nov. 28, 1944 

